Everybody knows intuitively what mental fatigue is. However, we poorly understand why fatigue emerges with time spent on demanding cognitive work and how such 'cognitive fatigue' impacts neural processing and behavioral guidance. Here, we review experimental investigations that induced cognitive fatigue and recorded its potential markers, including self-report, behavioral performance, economic choice, physiological and neural activity. We then review theoretical models of cognitive fatigue, classically divided into biological and motivational accounts. To explain key observations and reconcile debated theories, we finally propose a conceptual model (dubbed MetaMotiF), in which cognitive fatigue would emerge for biological reasons and yet affect motivational processes that regulate the behavior. More precisely, fatigue would arise from metabolic alterations in cognitive control brain regions, following their excessive mobilization. In turn, these metabolic alterations would increase the cost of cognitive control, which would shift decisions towards actions that require little effort and yield immediate rewards.